1. Field of the Invention
The present invention relates to a coaxial cable, and more particularly relates to a coaxial cable in which a central conductor is used for transmitting images. A first conductor and a second conductor, which is outside the first conductor, are provided on the circumference of concentric circles having the central conductor as their center.
2. Description of Related Art
For transmitting images, coaxial cables having characteristic impedance of 75 Ω (ohms) are generally employed. It is known that the attenuation of picture signals is phenomenally approximated by, roughly, one-half power of the frequency. The disclosed measured values, for example, in a 3C-2V coaxial cable: when the frequency of the picture signal is 4 MHz, the attenuation is 27 dB/Km; when the frequency of the picture signal is 6 MHz, the attenuation is 32 dB/Km; when the frequency of the picture signal is 8 MHz, the attenuation is 38 dB/Km; and when the frequency of the picture signal is 10 MHz, the attenuation is 42 dB/Km. According to these measured values, it is understood that, generally, the higher the picture frequency, the larger the degree of the attenuation. Meanwhile, various factors other than the characteristic impedance of the coaxial cable are conceivable as the cause of the attenuation of picture signals. Among those factors, an external factor, other than noises, caused by the picture signal itself causes noises. Noises caused by such external factor are known to be a cause of the attenuation of picture signals and disturbance of picture signals. Herein, it is considered that the disturbance or the like of picture signals because of noises caused by such external factor is included in a broad sense of attenuation.
Meanwhile, cable compensators are employed in accordance with needs to compensate the attenuation of the picture signals. Cable compensators are amplification means of picture signals and essential means for providing images influenced by as little attenuation as possible, or for compensating for the attenuation. These compensators are disposed, in advance, at a predetermined distant interval in accordance with the attenuation state of the picture signals, and at every position, picture signals are amplified. In a case of a general coaxial cable, one cable compensator is disposed about every 100 m.
However, when cable compensators amplify attenuated picture signals, a phenomenon is caused in which noises are amplified at the same time. In the case of noises, the next cable compensator further amplifies the amplified noises that have been amplified by one cable compensator. Amplification is repeated one to the next, and as a result, the attenuation of the picture signal in a broad sense is accelerated. As described above, cable compensators have to be employed in order to compensate for the attenuation of picture signals. However, on the contrary, when cable compensators are overused, the result is that clear images cannot be obtained because of noises.
In relation to such coaxial cable, the inventor of the present application has formerly disclosed a coaxial cable comprising a first tubular electro-magnetic shielding conductor and a second tubular electro-magnetic shielding conductor for feeding current for driving devices. In this coaxial cable, the first tubular electro-magnetic shielding conductor is provided outside a central conductor with an insulator therebetween, the second tubular electro-magnetic conductor is provided outside the first tubular electro-magnetic shielding conductor with an insulator therebetween, the outer circumference of the second tubular electro-magnetic shielding conductor is coated with an external coating which is an insulator, and the first tubular electro-magnetic shielding conductor and the second tubular electro-magnetic shielding conductor are configured for feeding current for driving devices. According to this coaxial cable, for example, when electricity is allowed to flow with the inner first tubular electro-magnetic shielding conductor being negative and the outer second tubular electro-magnetic shielding conductor being positive, the magnetism in the center space becomes zero. Therefore, no magnetic disturbance is imposed on the central conductor, and magnetic actions caused in the cable form a barrier thereby carrying out protection against disturbing electric waves coming in from outside. Therefore, for example, the picture waveform sent from a monitoring camera is not disturbed at all, and the image of a monitor is not disturbed and clear images can be obtained.
[Prior Document]
Japanese utility model registration, publication No. 3024770 (FIG. 1)
However, there has been a problem for practical use regarding, among external noises, the influence of, for example, high frequency noises having steep wave forms generated when an accelerator of an electric system of an engine, for example, a motorcycle is suddenly stomped on. Also, in regard to image transmission, there has been a problem for practical use regarding how far a distance clear images having no attenuation can be transmitted without employing cable compensators.
The present invention has been developed to solve the above described problems. Its object is to provide a coaxial cable that is able to completely shut out external noises and transmit picture signals not influenced by noises over a long distance.